Bright cast alloy, and composition

ABSTRACT

An improved gold alloy casting composition wherein sufficient aluminum and/or silicon is utilized in an amount sufficient to form a lustrous oxidation resistant surface coating during and after casting, the method of casting utilizing these components and the cast product. In addition a de-oxidant may also be utilized.

Unlted States atent 11 1 1111 3,769,006 Ingersoll Oct. 30, 1973 [5 1 BRIGHT CAST ALLOY, AND 1,580,44-3 4/1926 Shields 75/165 COMPOSITION 2,654,146 10/1953 Mooradian 75/165 X 2,747,971 5/1956 Hein 75/165 X [75] Inv nt C yd IL g s Tonawanda, 2,840,468 6/1958 Brenner 75/165 N.Y. 2,980,998 4/1961 Coleman et al.... 75/165 X 1 Assigneez Williams Gold Refining p y 3,613,209 10/1971 P1querez 75/165 X neon-Formed Buffalo Primary Examiner-L. Dewayne Rutledge [22] Filed: Jan. 27, 1972 Assistant ExaminerE. L. Weise [21] Appl No 221 2 AttorneyC0nrad Christel et a1.

57 ABSTRACT [52] US. Cl 75/165, 75/135, 164/34,

' 164/122 An rmproved gold alloy castmg compos1t1on wherem 51 1m. (:1. C22c 5/00 Sufficiem aluminum and/Or Silicon is utilized in 58 Field of search 75/165, 135 amount Sufficiem form a lustrous Oxidation resistant surface coating during and after casting, the [56] References cited method of casting utilizing these components and the UNITED STATES PATENTS cast product. In addition a de-oxidant may also be utilized. 1,339,009 5/1920 Van Allen 75/165 1,557,431 10/1925 Davignon 75/165 6 Claims, No Drawings BRIGHT CAST ALLGY, AND-COMPOSITION The present invention is directed to the prevention-of the forming of oxide coatingsduring casting of goldalloys. More particularly the presentlinventionis directed to the use of aluminum and/or silicon in'dental and jewelry gold alloys wherein copper, iron, tin, indium or tinlindium mixtures are utilized as-the hardening metal. Even further, in the present invention, a minor amount of silicon and/or aluminum is incorporated in the casting composition to provide for a glass-like coating on the surface of the cast product adjacent themold.

In dental and jewelry gold alloys, the usual hardening element is copper. However, in'some casesiron, tin, indium, or tin/indium mixtures are used as the hardening element. When these alloys are cast, e.g. into investment molds in the lost wax process, some of the alloy is partially oxidized during meltingaand casting procedures. In particular, it appears that at leastsome of the hardening component is oxidized. .The formation of e.q. copper oxide. produces a black deposit on the surface of the resulting casting which must be removed by e.g., pickling acid, grinding or sandblasting. After removal of the oxide coating, the dull finish-must then be polished in order to obtain a metallic luster on the product.

In order to improve casting processes, and eliminate the above noted problems, many attempts'have been made to provide casting compositions or processes whereby the cast, or otherwise molded product has no oxide coating. For example, U.S. Pat. No. 3,613,209 utilizes an oxidationinhibitingametal, inert' gas, and injection molding to produce a gold alloy which doesnot have an oxide coating after molding. However, the above process requires complexand expensive operations such as injection molding under high pressure and the use of inert gas around the melted alloy.

Further, in casting processes,- oxygenscavengers such as zinc and indium have been utilized. However, neither zinc nor indium eliminates the dissolved oxides, and neither has a significan't effect on the oxide coating of the cast piece.

Another method utilized to at. least minimize oxide coating in casting has been vacuum casting. However, the utilization of high vacuums is a relatively expensive operation and such operations have not' been wholly successfuLFurther, various fluxes-have been utilized to prevent the formation of oxides during the-casting but are normally left in the crucible'and thus have not been effective oxidation preventives during solidification and cooling of the alloy inthe mold. As a result, graphhe has been used in the mold to protect the casting during solidification and coolingj and-i's atleastsomewhat effective. However, difficulty arises in thatsome or all of the graphite is burnt out of the investment during the pattern elimination in utilizing, the lost wax" casting process.

One standard method :of minimizingoxide content in cast metal is to add to the melt, an element which has an higher oxidation potential than the element to be protected from oxidation. This is called de-oxidation of the melt and the oxide primarily ends in slag on the surface of the melt. This slag is made-up of deliberately oxidized metals, accidentally oxidized metals and slagforming fluxes. However, when such dex-oxidation is utilized by the inclusion of, for example zinc and/or indium in noble metal alloys, there. isadecrease in the oxide content of the cast alloy but the surface oxide is .still present unless the amount of additive utilized is so great as to alter the propertiesof the alloys drastically. Thus, the surface oxidemust still be removed prior to polishing.

Thus, the present invention has as one of its objects the protection of the cast piece from surface oxidation during the important period of solidification'and cooling of the alloy in the investment mold.

Another object of the present invention is to minimize still further the inclusion and and/or solution of copper oxide in the melt.

A still further object is to minimize the loss of copper or other hardening elements during. the melting and casting of the product.

The present invention has a further object the provision of a simple process which allows for the substantial, if not complete, elimination of the oxide coating on the surface of the cast product.

Still further the present invention has as its object the production of an alloy which, as cast, has a lustrous gold finish which may be utilized without polishing.

The present objects and others will become clear from the remainder of the present specification.

In the present'invention a gold alloy containing for example, up to 50 percent copper or other hardening elements is, in'the'melt form, provided with an amount of silicon and/or aluminum to provide for a surface coating on the finally cast product. Casting is done for example, by the lost wax process and an investment mold and then the product is cooled and removed. The resulting product has a glass-like surface coating which is lustrous in character.

In casting dental or jewelry gold alloys in accordance with the present invention, an alloy containing a hardening element, preferably an element such as up to 50 percent by weight copper, most preferably more than about 5 percent copper, as well as other known additivessuch' as silver, zinc, etc. and has provided additionally, silicon and/or aluminum in an' amount sufficient to produce the lustrous cast product of the present invention. The amount of aluminum and/or silicon required varies with the surface area of the mold. For example, when a spherical type mold or a tooth is molded, a smaller surface area is present than for example, when a grid structure is molded. Thus, more of the additive of the present invention is required to produce the lustrous finish of the present invention on a grid structure. However, generally from about 0.01 to about 2 percent by weight based on the total alloy content of aluminum, silicon or both'should be provided, preferably from about 0.04 to about 0.5 percent by weight is provided. I

Although any conventional hardening elements may be utilized, the preferred group includes copper, iron,

tin, indium, and mixtures of tin and indium. When copper is utilized it may be present in amounts up to 50 percent by weight of the alloy, preferably above about 5 percent by weight. When iron is utilized it should be present in amounts up' toabo'ut'l percent by weight, preferably from about-0.25 percent to 0.75 percent by weight. When tin and/or indium are utilized the total amount of hardener," either-separately if one is used or combined if both are used, should be up to 7 percent by weight, preferably about 3 percent to 6 percent by weight.

In addition other conventional elements such as silver, palladium and even platinum may be utilized.

Silber is the most common in alloys and is often present in amounts of up to about 30 percent by weight, preferably about 5 percent to 25 percent by weight when present.

The aluminum or silicon may be added to the melt or to a mixture of the metals as they are being melted in either the pure form, or as master alloys, e.g., alloyed with silver. The alloy of the present invention may also be provided in e.g., bar form, having been previously cast and if needed rolled. In this case the original casting provides the coating, and the subsequent casting also provides a coating. In any case when a master alloy is utilized, the time of addition of the master alloy is not important. All of the components for the desired product alloy can be placed in e.g., a crucible or other heating means and melted, they may be melted singularly and mixed or melted in one crucible in any serial order. However, when aluminum and/or silicon is added in its unalloyed form, it is preferred that the silicon and/or aluminum be added last.

The alloy melt is then cast by normal procedures, e.g., pouring into a wax investment mold and cooling by standard procdures. The temperature of the melt depends upon the alloy composition and is the same as standard conditions. Generally, however, temperatures from about l500 to about 2500F., preferably about 1650F to about 2200F are used. The mold is held at standard temperatures during the period prior to casting, usually about 900F. to about l600F., preferably about 1 lOF to l500F. However, it should be noted that more of the additive of the present invention is required as the mold temperature is increased. In the cooling of the casting, normal procedures are again followed. The mold may be air cooled, quenched or even air cooled to some extent and then quenched.

As to the particular alloy uses contemplated by the present invention, exemplary alloys for dental uses in accord with the present invention are those containing from about 9 to 18 percent copper, as is standard in the art. Further, as to jewelry, the wide variations standard in casting gold alloys for jewelry are possible. That is, normal jewelery alloys will contain from about 92 percent down to about 60 percent gold or lower, although other variations may be possible. 18K gold contains about 75 percent gold and 10 to 20 percent silver in combination with to 15 percent copper, while a 14K dental gold contains normally 63 percent gold, 30 percent silver and 12 percent copper. The process of the present invention, however, is not dependent upon the various alloy compositions in order to obtain the product of the present invention, but the criticality of the present invention is the provision of sufficient silicon and/or aluminum to produce the glass-like coating on the surface of the molded product and thus produce the lustrous product.

Further, as to other additional components, the deoxidants, such as zinc, magnesium, barium, may be used.

In utilizing the process of the present invention'to produce the bright as cast gold alloy of the present invention, the glass-like surface coating is produced. Microscopic studies, utilizing a scanning electron microscope, have shown that there is a glassy coating on the surface of the cast product in the area in contact with the mold. Further, the surface is not oxidized, but if it is ground or otherwise abraded to remove the surface coat and then subjected to oxidizing conditions,

oxidation will occur but only in the area where the abrasion is effected. The glass-like nature of the coating appears to be the best way of describing it, although at the present time its actual character is not known. However, it appears that it may be some form of a silicon or aluminum containing material. If the silicon or aluminum is oxidized as the oxidation potential would indicate, silica or alumina would be produced. However, these compounds melt at above 2675F. and 3650F. respectively depending upon their structure and thus it would appear that no liquid form of the silica or alumina would be produced. However, a glasslike coating could be formed at the time of the oxidation reaction. Further, some form of Eutectic may be formed. Therefore, the exact nature of the coating is not clear but it is clear from the experimental results hereinafter disclosed that significant improvements occur by the addition of the present additives in the amounts discussed and that a coating of some nature which is not subject to oxidation is produced.

The present invention will be further shown by the following examples.

EXAMPLE 1 When attempting to provide a melt that will not oxidize, it is common to use the free energy of oxide formations as a guide to which elements will be effective. This free energy of oxide formation can be obtained from standard reference books, such as The Handbook of Chemistry and Physics published by the Chemical Rubber Co. Exemplary free energies based on single oxygen atoms are as follows:

TABLE I Element 25C. I350K Calcium l44.4 lOl.6 Aluminum -l25.6 99.0 Copper 35.() -l7.8 Silver 4.8 +124 In these figures it can be seen that both calcium and aluminum, in their oxide forms, are in a much lower energy state than copper is in its oxide form. Therefore, it would be expected that both calcium and aluminum would oxidize in preference to copper and thus would make good deoxidants.

However, when a dental alloy containing 9.5 percent by weight copper as its only hardening agent (the alloy containing 25 percent silver and 3 percent palladium in addition to the copper and gold) had about 0.04 percent calcium added to it and was cast into an investment mold which was burned out at ll 00F, the resulting product had a gold color but no metallic luster.

On the other hand, when the same 9.5 percent copper containing alloy having about 0.04 percent aluminum instead of calcium was cast by the same method the resulting alloy had a gold color and was lustrous in the area in contact with the mold.

Example II A dental alloy containing 9 percent by weight copper, 12 percent by weight silver and 4 percent by weight palladium was cast after adding 0.02 percent lithium to the melt by pouring it into an investment mold wherein the mold temperature was llO0F. The resulting cast piece had no black copper oxide on its surface; but had no metallic luster.

EXAMPLE III A dental alloy alloy containing 9.5 percent by weight copper (as the alloy of'Example I) and 0.64 percent by weight titanium was melted and cast as in Example 11. The surface in contact with air during the solidification and cooling was blue-black, whilethe' surface'in contact with the investment during the solidification and cooling was gold in color but has no metallic luster.

EXAMPLE IV An alloy of the same composition as in Example III, except that 0.16 percent tantalum was substituted for the titanium, was cast in the same manner as Example III and the results were the same;

EXAMPLE V EXAMPLE VI Three casting melts with the 9.5 percent copper alloy as in Example I, but with 0.04, 0.08, and 0.228 percent by weight, barium were prepared and cast. In all three cases the resulting alloy product had a gold color in the area in contact with the investment, but no luster.

EXAMPLE VII Numerous castings, as set forth'in the table below, were prepared with the results as listed; the castings containing 9 percent and 9.5 percent copper are the same as above except for the aluminum or silicon content, while 18 percent copper casting also contains, 13 percent silver and percent palladium, the remainder being gold.

TABLE II Copper Element Results Content AddedWt% Casting Air Contact Portion 9 Al 0.01 Gold-no luster Black 9 Al 0.02 Gold-lustrous black 9 Al 0.04 Gold-lustrous Gold-lustrous 9 Al 0.08 Gold-lustrous Gold-lustrous- 9 Al 0. l 6 Gold-lustrous Gold-lustrous 9 Si 0.08 Gold-lustrous Gold & Black 9 Si 0.16 Gold-nor luster Gold-no luster 9 Si 0.40 Gold lustrous Gold-no luster 9 Si 0.84 Gold-lustrous Gold-lustrous 9.5 A1 0.04 Gold-lustrous Black 9.5 A1 0.08 Gold-lustrous Gold-lustrous 10% black 9.5 Al 0.16 Gold lustrous Gold-lustrous 10% Gray 9.5 Al 0.32 Gold-lustrous Gold-lustrous 9.5 Si 0.44" Gold-lustrous, Gold-no luster, few black spots 50% gray 9.5 Si 0.48 Gold-lustrous Gold-lustrous,

20 black spots 9.5 Si 0.56 Gold-lustrous Gold-lustrous, Gray spots I 8 Al 0.06 Gold-lustrous, Black 50% black 18 Al 0.08 Gold-lustrous Lustrous,

l0 black 1 8 Al 0. l 6 Gold lustrous Gold-lustrous 18 Si 0.16 Gold-no luster Black 18 Si 0.32 Gold-lustrous black &'black 18 Si 0.80 Gold-lustrous 50% black & no luster l8 Si 0.90 Gold-no 'luster Black From the examples it is clear that aluminum and silicon when used separately produce alloys that are lustrous as cast when very small amounts of either component are utilized. Further, when a more limited range of amounts is utilized the casting not only results in a lustrous surface in the area in contact with the mold, but also'results in a lustrous surface in the area which is not in contact with the mold. For example, when 0.04 to 0.15 percent aluminum was used in a 9 percent copper containing gold alloy the resulting casting was lustrous in the area in contact with the mold as well as the area in contact with air. Further, it appears that in addition to being lustrous the castings of the present inventionare more easily recovered from the investment mold. That is, there, appears to be no adhesion between the mold and the metal and thus the recovery is quite simple. This is possibly due to the lack of wetting of the investment by the molten alloy, although the reasons are not clear'at the present time.

EXAMPLE VlII An 18 percent copper containing metal alloy, as in Example VII, was cast with five different aluminumsilicon contents. Thecontents of the five different examples are set forth below.

TABLE III Trial Components Amounts A Al 0.12 Si 0.20 B Al 0.10 Si 0.10 C Al 0.03 Si 0.05 D Al 0.02 .Si 0.10 E- Al 0.02 Si 0.05

In all of the above cases the casting had a metallic luster where it was in contact with the investment and varying degrees of blackness where it was cooled in contact with air. From the previous examples it can be seen that where 0.08 percent aluminum is required to produce the lustrous casting, and a completely lustrous casting was not produced utilizing silicon, as little as 0.02 percent plus 0.05 percent silicon produced a completely lustrous casting except for the surface whichsolidified in air contact. Thus it appears that aluminum and silicon are more than additive in their effects.

EXAMPLE IX In gold basedental alloys containing, in addition to thegold, 3 percent palladium, 8 percent platinum, less than 1 percent indium and tin, and 0.25 percent by weight iron as the hardening agent, normally a dark oxide coating is formed when the lost wax process is utilized for casting. ,In accord with the present invention, however, varying amounts of aluminum were added to the casting melts with other results. When 0.01, 0.02, and 0.04 percent by weight of aluminum were'addedto three different casting compositions the resulting product was somewhat'lustrous butthe investment stuck to the surface of the cast product. However, when. 0.08, 0.16 and 0.32 percent aluminum were added the product was lustrous and was easily recovered from the investment.

EXAMPLE X When castings were made in accordance with Example lX, but silicon was used instead of aluminum, 0.02 percent silicon was required to obtain a low luster casting and only 0.08 percent silicon was required to obtain the full metallic luster.

EXAMPLE XI Further dental casting alloys were prepared in accord with Example IX utilizing tin and/or indium as the hardening agent. The casting composition was moditied with aluminum in amounts of from 0.04 to 0.32 percent by weight. The resulting castings were lustrous but the investment tended to stick.

EXAMPLE XII EXAMPLE XIII A 9 AK jewelry alloy (38.5 percent gold, 50 percent copper, and l 1.5 percent silver) was modified with aluminum with the following result:

% Aluminum Casting Results 0.02 Black and dull 0.04 Black and dull 0.06 Lustrous with black spots 0.08 Lustrous EXAMPLE XIV Silicon was utilized to modify the casting composition of Example Xlll with the following results:

% Silicon Casting Results 0.02 Black and dull 0.04 Black and dull 0.06 Lustrous with black spots 0.08 Lustrous EXAMPLE XV It is apparent from the scanning electron photomicrographs that adding silicon and aluminum to a melt results in a cast product having an oxygen imprevious coating or layer on the surface of the casting. This layer is continuous if there is sufficient additive, but discontinuous if insufficient additive (e.g. 0.06 percent Al or 40 ing element is iron.

Si in Examples XIII and XIV). The requirements for the amount of additive with regard to the surface area can be seen from the results wherein the dental alloy containing 18 percent copper used above was modified with 0.04 percent aluminum, 0.06 percent silicon, 0.03 percent chromium, and 0.01 percent yttrium. Two types of castings were made with the alloy composition, one being a sphere and the other a grid. The spherical structure obviously has a significantly lower surface area per unit volume than the grid and in these castings was lustrous. However, the grid, although it was quite lustrous showed small black spots which were evident under magnification. Thus the surface coating phenomenon of the present invention is seen and the requirements for sufficient amounts of aluminum and/or silicon to form this surface coating are evident.

What is claimed:

1. A gold alloy investment mold casting composition consisting essentially of from about 0.07 percent to about 0.32 percent by weight of silicon and aluminum,

said silicon being present in an amount of at least 0.05

by weight and said aluminum being present in amount of at least 0.02 percent by weight, a hardening element selected from the group consisting of copper, iron, tin, indium, and mixtures of tin and indium, said copper, when present, in amounts of 5 percent to 50 percent by weight of said alloy; iron, when present, in an amount of up to 1 percent by weight of said alloy; tin, when present, in amounts of up to 7 percent by weight of the alloy; indium, when present, in amounts of up to 7 percent by weight of the alloy; and both tin and indium combined, when present, in a total amount of up to 7 percent by weight of the alloy, the balance essentially gold.

2. The composition of claim 1 wherein there is additionally present about 0.04 percent by weight, based on the composition, of chromium.

3. The composition of claim 1 wherein said hardening element is copper.

4. The composition of claim 1 wherein said harden- 5. The composition of claim '1 further including a deoxidant selected from the group consisting of zinc, magnesium, and barium.

6. The composition of claim 1 wherein both tin and indium are utilized in a combined amount of 3 to 6 per cent by weight.

. :0: in a a: 

2. The composition of claim 1 wherein there is additionally present about 0.04 percent by weight, based on the composition, of chromium.
 3. The composition of claim 1 wherein said hardening element is copper.
 4. The composition of claim 1 wherein said hardening element is iron.
 5. The composition of claim 1 further including a deoxidant selected from the group consisting of zinc, magnesium, and barium.
 6. The composition of claim 1 wherein both tin and indium are utilized in a combined amount of 3 to 6 percent by weight. 